1. Field of the Invention
This invention relates to a process and apparatus for extracting mineral values from the earth. More specifically, this invention relates to a method and well system for recovering viscous hydrocarbons such as bitumen from a subterranean reservoir by injecting a heated fluid into the reservoir via an array of radially spaced horizontal wells.
2. Description of the Prior Art
In many areas of the world, there are large deposits of viscous petroleum, such as the Athabasca and Cold Lake Regions in Alberta, the Jobo Region in Venezuela and the Edna and Sisquoc Regions in California. These deposits are often referred to as "tar sand" or "heavy oil" deposits due to the high viscosity of the hydrocarbons which they contain. The distinction between tar sands and heavy oil is not settled. For both, however, normal reservoir flow rates are low, and techniques to improve flow are generally applicable to either without substantial changes including the present invention. Tar sand formations may extend for many miles and occur in varying thicknesses of up to more than 300 feet. These deposits may lie at or near the earth's surface or may be located under an overburden thousands of feet thick. However, tar sands not directly accessible from the surface consititute some of the world's largest presently known petroleum deposits. The tar sands contain a viscous hydrocarbon material, commonly referred to as bitumen, in an amount which ranges up to about 20% by weight. Bitumen is normally immobile at typical reservoir temperatures. For example, in the Cold Lake Region of Alberta, at a typical reservoir temperature of about 55.degree. F., bitumen or heavy oil is immobile with a viscosity of about one thousand poise. However, at higher temperatures such as temperatures exceeding 200.degree. F., the bitumen generally becomes mobile with a viscosity of less than 200 centipoise.
Since most heavy oil deposits are too deep to be mined economically, various in situ recovery processes have been proposed for separating the viscous oil from the sand in the formation itself and producing the oil through a well drilled into the deposit. Among the various methods for in situ recovery of bitumen from tar sands, processes which involve the injection of steam are generally regarded as the most economical and efficient. Steam can be utilized to heat and fluidize the immobile bitumen and, in some cases, to drive the immobilized bitumen toward production means.
The most common and proven method for recovering viscous hydrocarbons is by using steam stimulation techniques, an example being the "huff and puff" process. In this type of process, steam is injected into a formation by means of a well (the well is then sometimes shut-in to permit the steam to heat the bitumen), thereby reducing the viscosity of the crude oil. Subsequently, all formation fluids, including bitumen of reduced viscosity, water and steam, are produced from the same well using accumulated reservoir pressure as the driving force for production. Initially, sufficient pressure may be available in the vicinity of the wellbore to lift fluids to the surface; as the pressure falls, artificial lifting methods are normally employed. Production is terminated when the rate of oil production declines to an uneconomic rate. The cycle is then repeated until overall cycle economics are no longer attractive.
During the early cycles of steam injection and fluid production, oil production rates may be quite high since the oil nearest to the well is being produced. However, during subsequent steam cycles as the oil nearest the well is depleted, steam must move farther into the formation to contact the oil and as a result increased heat losses make the steam less effective as an oil recovery agent. The process looses efficiency with each cycle and eventually oil production becomes uneconomical.
Another general method for recovering viscous hydrocarbons is by using "thermal drive" processes. Such processes employ at least two wells, an injection well and a production well, spaced apart from each other by some distance and extending into the heavy oil formation. In operation, a heated fluid, usually steam or hot water, or a heat-generating fluid such as air, is injected through the injection well into the formation where it heats and drives fluids towards the production well. The dominant mechanisms for oil displacement are viscosity reduction of oil (as with steam stimulation), swelling and perhaps steam distillation, depending on the character of the oil. The principal advantage of using a thermal drive process is that higher overall recoveries can usually be obtained than with steam stimulation processes. For example, it has been the general experience in California that only relatively low recoveries are obtained overall by steam stimulation; on the other hand, while the recovery is higher with steam floods, more heat is used per barrel of oil produced.
One basic problem with thermal drive is that while the region of the reservoir which has been swept by the heated fluid or heat-generating fluid contains low residual oil saturations, generally less than half of the reservoir is swept before the flowing heated or heat-generating fluid breaks through at the production well. Once this occurs, fluid bypass tends to make further operations uneconomical. A great deal of work in the industry has been devoted to improving the fraction of the reservoir volume which can be swept by the thermal process before the heated or heat-generating fluid breaks through at the production well causing oil production to become uneconomic, i.e., improved sweep efficiencies are needed.
Many of the more promising approaches currently being developed utilize combinations of horizontal wells in order to improve oil recoveries. Such horizontal wells may also be combined with access shafts or very large-diameter boreholes capable of accommodating the passage of men and equipment.
One such process is described in U.S. Pat. No. 4,020,901 which issued on May 3, 1977 to Peter Pisio and Charles F. Kirkvolt (Chevron Research Company). This patent discloses a special arrangement of equipment for recovering heavy oil from very thick tar sand formations. The invention involves the use of heating conduits extending horizontally from a vertical shaft into the tar sand formation. Steam is flowed through the horizontal conduits to indirectly heat the adjacent formation and create a flowpath surrounding the conduits for production of fluids. In order to promote flow of heated petroleum to the flowpath, steam is injected directly into the formation via separate injection conduits. The patent discloses the use of a set of vertical injection conduits for delivering steam to the formation; alternatively, a set of slanted wells may be used to deliver the steam to the formation. In essence, the approach taken is to sweep the heated petroleum into a laterally extending flowpath and outwardly along the flowpath. The heated petroleum is then recovered near the end of the horizontal conduit and floats into the shaft. Presumably, over time, the steam will sweep more and more petroleum to the end of the horizontal conduit thereby creating an expanding steam saturated region.
One problem with the method of U.S. Pat. No. 4,020,901 is that while the creation of a flowpath is highly desirable, the approach here will be inherently extremely slow since heat is delivered to the formation for this purpose only indirectly. Conductive heating by indirect heating of the formation adjacent the horizontal conduit will be slow. Also, the method does not utilize many of the advantages of steam stimulation, such as removal of near-wellbore damage caused by fine solids, and asphaltic paraffinic deposits. In addition, a hot path between the injection location and production location is needed to effect fluid communication; however, this hot path of very high mobility tends to cause hot injected fluid to bypass the cold, viscous oil which must be heated.
Another patent utilizing a mine shaft or other large-diameter access shaft is U.S. Pat. No. 4,160,481 which issued on July 10, 1979 to L. J. Turk and R. O. Kehle (assignee: the HOP Corporation). Some popular accounts of this method have referred to it as the "hotplate" process. This is because steam is injected into the formation via horizontal wells which extend from a central access shaft like spokes on a wheel. The steam penetrates into the formation upwardly due to gravity and heat is transferred to the viscous oil. Once heated, the oil begins to flow. Oil and water are produced back through the horizontal wells and pumped through the vertical access shaft for treatment. The patent also discloses that oil may be more effectively swept from the formation by simultaneously injecting steam via one or more of the horizontal wells and producing oil via one or more other horizontal wells. However, the technique is primarily a steam stimulation or steam soak technique.
While U.S. Pat. No. 4,160,481 represents an advancement in systematically heating a formation and recovering normally immobile heavy oil in an efficient manner, problems still exist in the sweep efficiency of the process. It is believed that substantial quantities of oil will remain between the radially extending horizontal wells. Therefore, there is a continuing need for an improved thermal process for the effective recovery of viscous hydrocarbons from subterranean formations such as tar sand deposits, especially a process having improved sweep efficiency.